Power converter stations may comprise a valve unit formed of a stack (or column) of power converter modules. The converter modules may be connected in series. In a power converter module, transistors (or switches), such as insulated-gate bipolar transistors (IGBT), are typically associated with a capacitor adapted as energy storage component for the converter module.
The valve unit may also include a by-pass switch which, if a failure is detected at a converter module (such as for instance at one of the transistors of the converter module), provides another current path. In other words, when a failure is detected at a converter module, a control unit may cause the converter module to be by-passed (short-circuited). When the by-pass switch has been triggered, the energy stored in the capacitor of the converter module is totally discharged through the (closed or activated) by-pass switch or the failed transistor, which may create a very high current peak (in the range of for example hundreds of kilo amperes).
At the same time, the energy storage used in design of power converter modules tends to increase which in turn tends to increase the level of the current peak upon failure (at discharge). Fault handling related to increasing energy storage in power converter module design is one of the major challenges for future HVDC power stations.